Geopolymer formation processes at room temperature studied by 29Si and 27Al MAS-NMR

A systematic study of 29Si and 27Al MAS-NMR has been carried out in an attempt to understand the reaction mechanism of the geopolymer formation at room temperature. For selected compositions, reactions were followed at selected times after the initial mixing of the reactants until the final products formed. The reactants were silicate anions, metakaolin and aqueous alkali. Silicate anions from various sources, such as sodium orthosilicate (Na4SiO4) and two types of sodium silicate solutions (2SiO2·Na2O and 4SiO2·Na2O) were investigated. Further, silicate anions formed from the reaction of colloidal silica and aqueous alkali was also studied as a source for silicate anions. Interaction of silicate anions with Al sites of metakaolin soon after the mixing was obvious as 29Si MAS-NMR resonance lines of silicate anions were shifted to less shielded values. As the reaction progressed, the coordination of Al (IV, V, VI) in metakaolin changed almost completely to IV. It was indicated by NMR observation that polymeric silicate anions of high cross-linked densities were better than monomeric silicate anion in the geopolymerisation process.